The invention relates to a process of producing high-temperature resistant films of polyimide polymers, wherein a solution of the polymer is an organic solvent is caused to flow out through a slot spinneret of a film former and to coagulate in an aqueous precipitation bath containing the organic solvent. The film made up of coagulated liquid film is stripped, the organic solvent is removed and the film is dried.
As far as high-temperature resistant polyimide polymers go, primarily those may be employed which comprise structural units of the general formula ##STR1## wherein R represents 2,4- and/or 2,6-toluylene or is a group of the formula ##STR2## to produce the high-temperature resistant polymer, preferably benzophenone-3,3',4,4'-tetracarbonic acid dianhydride-phenyldiamine 4,4'-methylene-bis(phenylisocyanate) and 2,4- and 2,6-toluylene diisocyanate are used.
A process of the initially defined kind is known from U.S. Pat. No. 4,426,486. There, a liquid film formed in a slot spinneret of a heated, at least 40% solution of a polyimide polymer in dimethyl formamide is cast onto the surface of a cooled drum. The lower part of the drum is immersed into an extraction bath composed of dimethyl formamide and water; the film formed on the surface of the drum is stripped over a stripping means composed of rolls, the organic solvent is removed during the passage through an aqueous washing bath, and the film finally is dried.
During the film forming stage, the surfaces of the liquid film are in contact with the drum surface on the one hand and with the aqueous extraction bath on the other hand. Thereby, a transparent aquagel film containing about 30% water is formed.
In order to ensure sufficient strength of films produced in such a way, it is necessary to orient the aquagel film in the machine direction and transverse to the machine direction. Subsequently, the film is dried. The biaxial orientation of the film damages the marginal zone of the film; hence undesired waste results which must be separated in the form of a wide edge strip.
The invention has as its object the production of high-temperature resistant films of polyimide polymers, where both surfaces of the liquid film are in equal contact with the precipitation bath during formation of the film and no transparent aquagel is formed. Furthermore, the process aims at making possible the use of commercially available polymer solutions without further concentration. The strength properties are sufficient without orientation, and undesired shrinkage at elevated temperatures is avoided. Moreover, the process aims, by avoiding the uniaxial or biaxial orientation stpe, which leads to an unavoidable loss in the thickness of the film, at producing a thinner and, thus, more readily extractable primary film.
According to the invention, liquid film formed in the slot spinneret is allowed to freely fall directly into the aqueous precipitation bath through a short air gap. The film formed is drawn off the precipitation bath, is subjected to subsequent thermal treatment. The organic solvent is then rermoved by treatment with an aqueous extraction agent, and the film is finally dried.
In this procedure, the white opaque film formed in the precipitation bath is converted into a transparent golden-yellow film by thermal treatment.
Preferably, the thermal treatment of the film formed in the precipitation bath is carried out under supply of hot air at a temperature of 40.degree. to 130.degree. C.
According to a preferred embodiment, the extraction of the thermally treated film is effected in steps, at first with a bath consisting of organic solvent and water and subsequently with pure water, at a temperature of 70.degree. C. to 100.degree. C.